1. Field of the Invention
The present invention relates generally to a method for repairing components of a mud motor of the type used in the earth drilling industries and, more specifically, to a method of repairing a worn rotary connection of a rotor using a stub welding technique.
2. Description of the Prior Art
Mud motors are used for various drilling tasks such straight hole, horizontal and directional oilfield drilling, river crossings and other utility drilling. They are connected to the drill string to rotate and steer the drill bit. Rotation is provided by a power section, which is typically a positive displacement motor that is driven by drilling fluid circulation. Axial and radial drilling loads are directed to the drill string by the bearings within the bearing assembly.
In a typical horizontal drilling application, surface pumps are used to circulate the drilling fluid to flush rock cuttings to the surface for disposal. The drilling fluid flows down through the bore of the drill string, exiting into the annulus of the well through the jets in the drill bit. The cuttings are flushed up the annulus of the well by the returning drilling fluid. The annulus pressure is substantially lower than the drill string bore pressure due to the pressure drop that occurs as the drilling fluid passes through the drill bit jets.
The typical mud motor is a progressive cavity positive displacement pump (PCPD). As mentioned, the PCPD pump uses drilling fluid to create eccentric motion in the power section of the motor which is transferred as concentric power to the drill bit. Based on the principle developed by Rene Moineau, the theory states that a helical rotor with one or more lobes will rotate eccentrically when the stator contains more lobes than the rotor. The mud motor uses different rotor and stator configurations to provide optimum performance for the desired drilling operation. In certain applications, compressed air, or other gas, can be used for mud motor input power.
Normal mud motor construction consists of a top sub, which connects the mud motor to the drill string; the power section, which consists of the rotor and stator; the transmission section, where the eccentric power from the rotor is transmitted as concentric power to the bit; the bearing assembly which protects the tool from off bottom and on bottom pressures; and the bottom sub which connects the mud motor to the bit.
A mud motor is usually described in terms of its number of stages, lobe ratio and external diameter. Stages are the number of full twists that the stator makes from one end to the other and the lobe ratio is the number of lobes on the stator, to the number of lobes on the rotor (the stator always has one more lobe than the rotor). The operating parameters include flow rate, bit rpm and torque. The relationship between the rotor and the stator geometry determines the rotational speed and torque. The rotational speed is proportional to the flow rate and torque is proportional to the pressure drop in the fluid as it flows through the motor. These principles will be familiar to those skilled in the relevant arts.
Hunting Energy Services, Houston, Tex., and Casper, Wyo., is a leader in progressive-cavity, positive displacement mud motors. Typical sizes range from 1 11/16″ to 9⅝″, for various applications, be it coil tubing or large hole applications. These include both straight hole and directional applications. Power sections are available in a wide range of speeds.
There are various conditions in which the internal components of the mud motor can be damaged in use. For example, when the bit is bottomed and the motor is effectively working, there is a notable increase in the pressure in the fluid system. This is caused by a restriction within the motor. If the differential pressure is too high then the motor can stall which means the bit has stopped turning and this can cause severe damage to the internal surface of the stator. Another particular problem deals with the “rotary connection” at one end of the rotor which is used to connect the rotor body to the mating internal components of the motor. Sometimes, either due to normal use or misuse, the rotary connection can be damaged so that it needs to be replaced.
The existing “stub welding” techniques used in the industry to repair or replace damaged rotary connections do not work adequately. When using, for example, a 17-4 stainless steel material, it is not generally possible to get the rotary connection back to an acceptable mechanical condition. As a result, it may not be possible to return the motor to service for multiple trips or uses.
As a result, a need exists for a technique for reconditioning the rotor of a drilling mud motor, which technique preferably uses an improved stub welding technique to repair the rotary connection of the rotor.